Shiga toxin (Stx), also known as Shiga-like toxin (Slt), is a class of bacterial exotoxins having enterotoxicity, cytotoxicity and neurotoxicity. It is produced by enteric pathogens and divided into type-1 and type-2 (Stx1 and Stx2). Stx, as the key virulence factor of enteric pathogens such as Shigella dysenteriae, Vibrio cholera and Escherichia coli, may lead to several serious complications such as hemorrhagic colitis (HC), thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) which has a high fatality rate. Both of epidemiological and clinical data have demonstrated that Stx2 is more correlated with infectious complications especially HUS than Stx1 and thus regarded as the target for drugs.
At present, as for the infection caused by Stx-producing pathogens, there is not any specific and effective prophylactic or therapeutic drug for the first-aid treatment. Antibiotics commonly used in clinical treatment have given rise to the emergence of multiple drug-resistant bacterial strains, making such therapies noneffective. More seriously, the use of antibiotics may trigger the excessive release of Stx due to the rupture of bacteria, further increasing the risk of developing complications. Therefore, antibiotics should be used with cautions in the treatment of Stx-associated diseases and a specific drug should be developed as a novel effective method.
Stx2 (also known as Slt2) is composed of a single A subunit (Stx2A) and five B subunits (Stx2B). The Stx2B binds to globotriaosylceramide (Gb3) receptor on the surface of eukaryotic cells, and then Stx2A binds to 28S rRNA to cause cytopathy. Therefore, the binding of Stx2B to receptor Gb3 is the initial key step for toxicity. If such binding is blocked, the toxicity of Stx2 can be substantially inhibited. Studies have been mainly focused on the toxin receptor analogues and antibody therapy. Nishikawa reported that polysaccharide compounds which could bind to Stx had a potential therapeutic value (Nishikawa K, Matsuoka K, Watanabe M, et al., Identification of the optimal structure required for a Shiga toxin neutralizer with oriented carbohydrates to function in the circulation. J Infect Dis, 2005, 191: 2097-2105). Natori Y. also reported a treatment case associated with receptor Gb3 in 2002 (Natori Y. New drugs that prevent cytotoxicity of Shiga toxins. Nippon Rinsho. 2002, 60(6): 1131-1137) involving a new preparation that prevents the spread of toxin in gastrointestinal tract and a water-soluble preparation which suppresses Stx cytotoxicity in the circulation. However, the complex preparation process and side-effects of receptor-based polysaccharide compounds have restricted their development. Recently, some researchers reported that probiotics expressing a mimic of the receptor could be used in the treatment of Stx-associated diseases due to its significant neutralizing effect on the toxin (Paton J C, Rogers T J, Morona R, Paton A W. Oral administration of formaldehyde-killed recombinant bacteria expressing a mimic of the Shiga toxin receptor protects mice from fatal challenge with Shiga-toxigenic Escherichia coli. Infect Immun 2001, 69: 1389-1393). However, whether this approach is safe or not needs to be further tested. As for the antibody therapy, the monoclonal antibody reported by Sheoran et al. can block the binding between Stx2 and cell receptors and further neutralize the toxicity of Stx2 (Sheoran A S, Chapman-Bonofiglio S, Harvey B R, et al. Human antibody against shiga toxin 2 administered to piglets after the onset of diarrhea due to Escherichia coli O157: H7 prevents fatal systemic complications. Infect Immun 2005; 73:4607-4613). However, it can be clinically used only if preparation technology of antibodies is improved.
As a drug, a peptide compound is gaining more and more attentions due to its technical advantages such as the small molecular weight, relatively simple structure, mass production with low costs by chemical synthesis or genetic engineering methods, identified functions, low immunogenicity, slight side-effects, high security, diversity of administration routes and easy absorption. Thus, small-molecule peptides have become one of the most important sources for drug screening (Yuexi Li, Peitang Huang. Application of polypeptides in biological drugs and diagnosis reagents. Chinese Journal of Biochemical Pharmaceutics, 2001, 22 (4): 208-210). It is feasible to use a phage peptide library which contains a large quantity of peptides to screen for polypeptides having inhibitory effects on a toxin. However, peptide-based inhibitor against Stx has been rarely reported recently. The related literatures are mainly studies of Nishikawa K in 2006 (Nishikawa K, Watanabe M, Kita E, et al. A multivalent peptide library approach identifies a novel Shiga toxin inhibitor that induces aberrant cellular transport of the toxin. Faseb J 2006; 20:2597-2599) and studies of Miura Y (Miura Y, Sakaki A, Kamihira M, Iijima S, Kobayashi K. A globotriaosylceramide (Gb3Cer) mimic peptide isolated from phage display library expressed strong neutralization to Shiga toxins. Biochem Biophys Acta 2006; 1760:883-889).